The present invention relates in general to a method for joining metal parts, and specifically to a method for joining metal parts at least one of which is composed of a beryllium-aluminum alloy.
Beryllium-Aluminum alloys provide an advantageous material for a number of applications, including uses in aerospace and electronics. The low weight and high stiffness of these alloys allow use of this material in these applications. These alloys are characterized by their high fatigue strength, high heat capacitance, moderately low coefficient of thermal expansion, high thermal transfer, high stiffness, and low weight.
One present problem in the use of beryllium-aluminum alloys (Bexe2x80x94Al alloys) is the difficulty in joining parts made of this alloy. Bexe2x80x94Al alloys have a surface oxide layer, which makes it difficult to join or coat this surface. To deal with this problem, prior attempts to join Bexe2x80x94Al alloy parts have used mechanical joining, adhesive bonding, or dip brazing. Mechanical joining (e.g. by threaded attachments, nuts and bolts) may localize the strength of attachment, add significant weight to the structure and provide a more uneven thermal profile. Adhesive bonding of Bexe2x80x94Al parts does not provide sufficient mechanical strength for many applications, and does not provide a uniform media for thermal transfer, and does not provide uniform material for thermal expansion.
Dip brazing of Bexe2x80x94Al parts requires that these parts be precisely constructed to form narrow faying surfaces and small joint gaps at the joined surfaces to retain the brazing alloy in the joint gaps during the brazing process. To form a moderate to complex assembly, it is difficult to manufacture parts that would allow adequate braze fillet formation. Braze fillets are advantageous in Bexe2x80x94Al part assembly where narrow faying surfaces exist because joint stresses are carried by these fillets, rather than by the narrow faying surfaces. In addition, dip brazing subjects the Bexe2x80x94Al parts to the conditions of the molten dip bath. This process can result in distortion of the part shape, or alternatively can produce substantial deposits of entrapped flux, further adding to part weight.
One method to join Bexe2x80x94Al parts is set out in U.S. patent application Ser. No. 08/939,762. In this method the surfaces of Bexe2x80x94Al parts are cleaned and placed together to form a part assembly. A braze alloy is placed at the joint locations and a braze flux is applied to braze alloy. In one embodiment this braze flux contains aluminum fluoride and lithium fluoride (for example in the range of 2-54% and 2-69% respectively). Alternatively, the braze flux may contain potassium chloride, aluminum fluoride, sodium chloride, sodium fluoride, and lithium fluoride.
One solution to this problem, disclosed in U.S. Pat. No. 5,470,014 has been to treat the Bexe2x80x94Al alloy part with an ion vapor deposition of aluminum to enrich the surface with aluminum atoms. The aluminum-enriched surface may then be joined to another aluminum rich surface using conventional means, such as vacuum brazing. An alternative is disclosed in U.S. Pat. No. 5,615,826. In this method, an aluminum alloy is coated onto a beryllium surface, as by gas metal arc welding. The beryllium surface coated with aluminum is then brought into contact with another beryllium surface. The aluminum alloy is then welded to attach the two beryllium surfaces together. The aluminum surface may be subjected to elevated temperatures and pressures to reduce porosity before the parts are attached together. The aluminum alloy coated beryllium may be shaped into a desired welding joint configuration prior to welding. All of the above patents are incorporated by reference in their entirety.
It would be advantageous to have an alternative method for attachment Bexe2x80x94Al alloy parts that provides a uniform, high strength attachment of such parts.
The present invention is a process for the joining of Bexe2x80x94Al alloy parts. The beryllium component of a Bexe2x80x94Al alloy part is selectively removed from a surface. This selective removal, such as by acid etching, produces an aluminum enriched surface. This aluminum-enriched surface may then be attached to another aluminum-enriched surface by conventional aluminum attachment methods. Fluxless vacuum brazing of the aluminum enriched surfaces is one useful method for attachment of parts.